For me, plank pose really sets the tone of your sun salutations, so it's worth spending some time to make sure you are set up properly. If you have SI issues and you naturally tend towards an anterior pelvic tilt (or as Nadine calls it, "sexy back" - LOL), then plank pose is another pose where you want to tuck the pelvis anteriorly. The best way I have found of 'feeling' this is to think about tucking the tailbone while hugging your belly button up towards your spine and holding it there. It's useful to employ the abdominal muscles to stabilise the pelvis in a neutral or anterior tilt, because if you then keep the belly muscles engaged as you start your transition, your pelvis will most likely stay in the right position as well. (NB: If you have a fairly flat lumbar curve or you naturally tend towards an anterior pelvic tilt, you can just focus on keeping the spine long and the abdominals and legs engaged. Don't try to tuck your tailbone too far in the other direction!)
From plank pose, the next tricky bit is to maintain the pelvic tilt as you come down towards the floor. This is quite challenging because you are fighting against gravity. It takes a fair amount of muscular engagement from the arms, abdominals and legs to get this stability in chaturanga dandasana, and probably years of practice! If you are having SI issues or lower back pain, I recommend working with your knees on the floor until you are able to keep your back stable. Doing it this way still builds a lot of arm and abdominal strength, but makes the transition much easier on the SI joint because it is easier to keep your pelvis tucked and your back "flat" as you come down.
Finally, work on coming up into the backbend. Here again, if you have SI issues or are experiencing lower back pain, I recommend working with cobra pose instead of upward facing dog. A super-safe way of doing cobra pose is to perform the pose with your hands lifted off the floor, like this:
With your hands lifted off the floor, you have to use your abdominal and back muscles at the same time to lift your chest, meaning that the SI joint stays nice and stable. Resist the temptation to lift the chest higher by pushing the hands into the floor and hyper-bending the SI joint!! That is a sure recipe for lower back pain in the long run!
If your back is feeling OK and you are comfortable keeping the abdominals engaged and the pelvis tucked, then work on coming into upward facing dog. Remember, keep the abdominals firm, the pelvis tucked, and the legs strongly engaged in this pose. This will help to reduce the angle of the bend in the SI joint and help you get a longer, smoother curve in your lumbar spine, as you can see in the pictures below!
How do you practice (or teach) this transition?